Zirconium oxide, also referred to as zirconia or ZrO.sub.2, undergoes a stress induced phase transformation within its crystalline structure in the temperature range of about 200.degree. C. to about 250.degree. C. Stresses associated with propagating crack fronts within the zirconium oxide provide the driving force for the phase transformation from the metastable tetragonal phase to the stable monoclinic phase. This phase transformation from the metastable tetragonal phase to the stable monoclinic phase results in about a four percent volume increase within the crystal lattice of the zirconia material. A stable cubic phase, which does not undergo a phase transformation, is also present within the crystal lattice of the zirconium oxide.
It is generally known that the metastable tetragonal phase may be retained at lower temperatures by combining yttrium oxide, also referred to as yttria or Y.sub.2 O.sub.3, in the range of about 2.0 to 2.8 mole percent, with the zirconia crystals--hence the common nomenclature yttria stabilized zirconia. Retention of the metastable tetragonal phase is further maximized by maintaining the grain size of the zirconia below a critical grain size value. The critical grain size is defined as that grain size below which the transformation from tetragonal phase to monoclinic phase is thermodynamically unfavorable. If the grain size is maintained below the critical grain size value, the crystalline matrix constrains the transformation within the zirconia crystals and the metastable tetragonal phase is retained. Therefore, the low temperature stability of yttria stabilized zirconia is directly related to the amount of metastable tetragonal phase which does not transform to the stable monoclinic phase, and is optimized when the yttria stabilized zirconia is characterized by grain sizes smaller than the critical grain size value, i.e., a fine microstructure.
The microstructure of the yttria stabilized zirconia material is related to the sintering process, in particular the sintering temperature at which densification of the powder occurs. It is generally known that as the sintering temperature of a material is reduced, the resulting grain size of the material becomes smaller and therefore a finer microstructure is produced. Two means for reducing sintering temperatures of the yttria stabilized zirconia powders are: (1) the use of ultrafine powders and (2) the use of liquid phase sintering techniques. Typically, zirconia powders are sintered in the temperature range of 1500.degree.-1700.degree. C. With improved processing techniques and ultrafine powder, the sintering temperature for this material has been reduced to about 1450.degree. C. However, using ultrafine ceramic powders is generally considered impractical on a mass production scale.
Liquid phase sintering produces rapid densification of the ceramic powders at low temperatures and has been used to effect low temperature densification of ceramic powders. A liquid phase sintering aid, comprising a reactive compound which forms a low viscosity liquid phase at relatively low temperatures, is utilized. The liquid phase sintering aid is required because the driving force for densification of the powders is derived from the capillary pressure of the liquid phase located between the fine solid particles. Requirements for liquid phase sintering are: (1) a sufficient amount of the liquid phase component, (2) homogeneous distribution of the liquid phase component, on a microscale, throughout the powdered mixture, (3) high solubility of the solid powdered component in the liquid component, and (4) appreciable wetting of the solid by the liquid, i.e., a low dihedral angle of wetting. In general, the amount of liquid phase should not exceed about 25 percent of the total volume and typically, significantly lesser amounts of liquid phase are sufficient.